The present invention relates to fluid ejection devices and, more particularly, to proximate positioning of drive circuitry with respect to heating elements of fluid ejection devices.
In a printhead of an ink jet printer, a drive bubble is formed with heated fluid or ink that causes a droplet of fluid to be ejected from a nozzle or orifice of a printhead towards the media. The fluid is heated by resistors that are activated in response to associated transistors. The resistors and transistors are often formed over a silicon substrate.
In some MOS transistors that may be used to fire a resistor, polycrystalline silicon, also known as polysilicon, is layered over the thermal isolation underlayer and is used as a high resistance, not quite insulating, conductor that acts as the gate of the transistor. When current is passed through the transistor gate, an electric field is established which xe2x80x9copensxe2x80x9d the flow of electrons between the source and the drain of the transistor, establishing a circuit. When current is turned off to the transistor gate, the electron flow stops, turning off the transistor.
A very thin thermal isolation underlayer, for example a silicon oxide layer, is often applied to the silicon substrate of the printhead, lying between the heating resistors and the silicon substrate. The underlayer protects the silicon substrate during the firing pulse of the resistor. Because the thermal isolation underlayer is often very thin, an electric field generated by the gate can influence the movement of the electrons in the transistor.
Often, the drive transistors have been located a distance from the resistors to protect the transistors from being exposed frequently to high heat, and thus shortening the operating lives of the transistors. Another reason for the distance between the transistors and resistors may be to minimize the mechanical pounding of the drive transistors by the explosions of the fluid bubbles when the fluid is heated.
A fluid ejection device or printhead, and a method of forming such devices, are described. In one embodiment, the printhead includes a firing chamber from which heated fluid is ejected. The printhead also includes a resistor that heats fluid in the firing chamber, the resistor formed in a substrate underlying the firing chamber. The printhead further includes a transistor electrically coupled with the resistor, the transistor also formed in the substrate. The transistor is positioned proximate to the resistor and at a distance within 60 microns thereof. The substrate has a width that corresponds to the distance between the resistor and the transistor.